1. Field of the Invention
The present invention relates to a method and apparatus for direct visualization and delivery of laser energy to a site that is difficult or inaccessible to reach. In particular, this invention relates to a method and apparatus for visualizing and delivering laser energy to site in a body passage.
2. Description of Related Art
Atherosclerosis, which is a major cause of cardiovascular disease, resulting in heart attacks, is characterized by the progressive accumulation of fatty deposits, known as plaque, on the inner walls of the arteries. As a result, blood flow is restricted and there is an increased likelihood of clot formation that can partially or completely block or occlude an artery, causing a heart attack. Arteries narrowed by atherosclerosis that cannot be treated effectively by drug therapy are typically treated by medical procedures designed to increase blood flow, including highly invasive procedures such as coronary artery bypass surgery and less invasive procedures such as balloon angioplasty, atherectomy and laser angioplasty.
Bypass surgery involves opening the patient's chest and transferring a vein cut from the patient's leg to the heart to construct a detour around the occluded artery. Bypass surgery requires prolonged hospitalization and an extensive recuperation period. Furthermore, bypass surgery also exposes the patient to a risk of major surgical complications.
Balloon angioplasty is a less invasive and less costly alternative to bypass surgery and is performed in a hospital cardiac catheterization laboratory by an interventional cardiologist. In this procedure, a balloon-tipped catheter is inserted into a blood vessel through a small incision in the patient's arm or leg. The physician uses a guide catheter to feed the balloon through the patient's blood vessels to the occluded artery. At that point, a guidewire is inserted across the deposits of atherosclerotic plaque, known as lesions, to provide a pathway for the balloon catheter. The deflated balloon is advanced over the guidewire, positioned within the occluded area and inflated and deflated several times. This inflation and deflation usually tears the plaque and expands the artery beyond its point of elastic recoil. Thus, although no plaque is removed, the opening through which blood flows is enlarged.
Atherectomy employs a rotating mechanical device mounted on a catheter to cut and remove plaque from a diseased artery. Although atherectomy, unlike balloon angioplasty, removes plaque from coronary arteries, existing atherectomy devices are not effective in treating certain types of lesions.
Laser angioplasty removes plaques by using light, in varying wavelengths ranging from ultraviolet to infrared, that is delivered to the lesion by a fiberoptic catheter. Early attempts to develop a laser angioplasty system used continuous wave thermal lasers that generated heat to vaporize plaque. These laser systems caused charring and significant thermal damage to healthy tissue surrounding the lesion. As a result, thermal laser systems have generally been regarded as inappropriate for use in the coronary arteries. In contrast, excimer lasers use ultraviolet light to break the molecular bonds of atherosclerotic plaque, a process known as photoablation. Excimer lasers use electrically excited xenon and chloride gases to generate an ultraviolet laser pulse with a wavelength of 308 nanometers. This wavelength of ultraviolet light is absorbed by the proteins and lipids that comprise plaque, resulting in precise ablation of plaque and the restoration of blood flow without significant thermal or acoustic damage to surrounding tissue. The ablated plaque is converted into carbon dioxide and other gases and minute particulate matter that can be easily eliminated by the body's circulatory system.
In laser angioplasty, conventional light guides using fiber optics are used to direct laser energy onto arterial plaque formations to ablate the plaque and remove the occlusion. Individual optically conducting fibers are typically made of fused silica or quartz, and are generally fairly inflexible unless they are very thin. A thin fiber flexible enough to pass through a lumen having curves of small radius, such as through arterial lumens from the femoral or the brachial artery to a coronary artery, typically projects a beam of laser energy of very small effective diameter, capable of producing only a very small opening in the occlusion. Moreover, the energy is attenuated over relatively small distances as it passes within a thin fiber. Small diameter fibers can mechanically perforate vessels when directed against the vessel wall as they are passed within the vessel toward the site.
In order to bring a sufficient quantity of energy from the laser to the plaque, light guides proposed for use in laser angioplasty usually include a number of very thin fibers, each typically about 100 to 200 microns in diameter, bundled together or bound in a tubular matrix about a central lumen, forming a catheter. Laser energy emerging from a small number of fibers bundled together produces lumens of suboptimal diameter which can require subsequent enlargement by, for example, balloon dilation. Such devices do not remove an adequate quantity of matter from the lesion, and their uses are generally limited to providing access for subsequent conventional balloon angioplasty.
Although individual fibers of such small dimensions are flexible enough to negotiate curves of fairly small radius, a bundle of even a few such fibers is less flexible and more costly. Coupling mechanisms for directing laser energy from the source into the individual fibers in a light guide made up of multiple small fibers can be complex. Improper launch of the laser energy into such a light guide can destroy the fibers. The directing of laser energy thus far has been limited to two-dimensional imaging with fluoroscopy. Frequently, it is not possible to distinguish whether the laser catheter is contacting plaque, normal tissue, or thrombus--all of which have very different therapeutic consequences as well as possible adverse side effects.
To facilitate laser angioplasty, optical scopes, known as angioscopes, have been proposed to directly visualize the area to be treated with the laser. Typically, the scope is inserted into the artery or vein through an incision and then periodically advanced to view desired locations along a length of the vessel. The scopes are attached to a viewing port to which an optical image is transmitted to be viewed by the physician.
Thus, it is highly desirable to combine the viewing function of an angioscope with the ability to deliver laser energy to a particular site simultaneously. One such attempt is disclosed in U.S. Pat. No. 4,641,912 to Goldenberg entitled "Excimer Laser Delivery System, Angioscope and Angioplasty System Incorporating the Delivery System and Angioscope." Goldenberg discloses a system which delivers excimer laser energy, by way of an optical fiber having a core of pure silica aided by an angioscope, to the desired target. U.S. Pat. No. 4,848,336 to Fox et al. shows a similar, albeit more complicated system. There are, however, several drawbacks to such a system.
For example, to ablate the target, the optical fiber must actually contact the target. This is undesirable because it may result in mechanical injury to the tissue which can have a deleterious effect upon the vessel wall. Moreover, contact with the tissue will probably obscure the angioscope's viewing of the target and thus render this method ineffective. Additionally, tissue contact with a laser catheter may produce intrinsically poor laser effects through spatial confinement of the ablation products as well as confining or producing a tamped mode where the laser-induced pressure waves or cavitation effects are magnified. This results in shattering or tearing the tissues, causing unwanted and potentially dangerous damage to the vessel wall.
In addition, an optical fiber, which is large enough to transmit sufficient laser energy to the target, may not be flexible enough to be directed through the particular body lumen to the occluded area. Moreover, it is limited to the delivery of ultraviolet Excimer Laser energy, when it is desirable to deliver a wider spectrum of laser energy, such as visible light for laser thrombolysis. And furthermore, it fails to disclose a means for protecting the eye or CCD crystals from the reflected laser light.
European Patent Application No. 87304072.9 to Tohru entitled "Laser Catheter" discloses a fluid core laser catheter. Tohru's laser catheter has several drawbacks. First, a catheter as described by Tohru can only transmit laser energy effectively less than 40 cm. This is because after 40 cm, the laser energy is too attenuated by scattering and bending losses. Second, to abate the target, the distal end of the catheter must actually contact the target. Thus, even if Tohru's catheter could be combined with Goldenberg's device, it would not be operable. The physician could not properly illuminate or view the target. Moreover, Tohru fails to disclose a means for protecting the eye or CCD crystals from reflected laser light.
Gregory et al, in their article "Liquid Core Light Guide for Laser Angioplasty" IEEE Journal of Quantum Electronics, Vol. 26, No. 12, December 1990, discloses a fluid core laser. Gregory et al. fail, however, to teach or suggest how their device could be operatively combined with conventional angioscopes, as taught by Goldenberg for example, to overcome the problems discussed above. Accordingly, a need remains for an instrument that avoids the drawbacks of conventional lasers and laser-angioscopes.